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Shopping questions are off topic according to the faq.
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BaarnJan 20 '13 at 16:56

I edited the question to make it less of a shopping request and more general. If you feel unhappy with the changes feel free to revert them or edit again.
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BaarnJan 20 '13 at 18:14

4 Answers
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For me, the gold standard is whether I can use the device to time the Tabata protocol. That protocol is 8x(20s exercise, 10s rest). The most common exercise is sprinting. This is a difficult protocol to time. Your device must:

Be programmable to beep alternating on 20s,10s intervals.

Loud enough to hear when your heart is pounding and the wind is rushing by or the gym music is blaring and you feel like you're about to explode. It should be annoyingly, painfully loud.

Small enough and light enough to carry conveniently while sprinting. It must strap down securely. It obviously needs to be wearable because you're running. If you set it on the ground and run away from it for 20s, you won't hear it beep.

Ideally it should be programmable to give a 3,2,1 countdown at the end of the rest interval.

The other difficulty with timing devices is accidental activation of the device. This happens frequently with watches if you are doing HIIT with barbell work or exercises with wrist hyperextension such as pushups or handstand pushups. Then your left hand frequently presses against the right side of the watch where the timing control buttons usually are. I have never found a watch that conveniently times my crossfit wods, and smartphones are too large and fragile for me to consider wearing one while in the middle of a high intensity workout.

Are you specifically seeking a "stopwatch", a single-purpose tool, a watch with interval capabilities, or a smartphone app (my recommendation)? Are you looking to track intervals, in which case anything with lap counting will do, or be notified at interval changes? Do you have simple, or complex, interval plans?

I current using Impetus for Android. It can set up complex interval programs easily through its UI, and if you can generate your own XML, you can improve over its UI. I currently get my XML from my stealth startup's DB based on a variety of training criteria.

The MotoACTV can do intervals, although not with the complexity of Impetus. It's a great dedicated device, though, modulo some dumb decisions on Moto's part.